Author Topic: Big 10 and Heavy Duty information  (Read 7076 times)

Offline Captkaos

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Big 10 and Heavy Duty information
« on: April 03, 2014, 09:33:13 PM »
This was written by a personal friend of mine KIILew or Ken Lewis (walking 73-87 encyclopedia).For those wondering this was about Big 10 and Heavy duty information, it was originally posted here by KIILew: http://67-72chevytrucks.com/vboard/showpost.php?p=1159345&postcount=8Some Big 10 history:

The Big 10 was promoted as a heavy duty two wheel drive half ton for the 1975 through 1980 model years. Although it offered truck buyers somewhat more load carrying ability than the standard C10 half tons, its real advantage- -and the motive behind its conception- -was that it provided buyers with the opportunity to purchase a half ton truck that was unencumbered by the dreaded catalytic converter, which was first introduced to a skeptical American car buying public just in time for the 1975 model year.

For several years prior to that time, all trucks with gross vehicle weight ratings of 6,000 lbs. and below were forced to comply with the strict “light duty”¯ emission standards that also affected passenger cars. The EPA’s selection of the 6,000 lb. threshold likely reflected its awareness that that number had become the traditional de facto dividing line between half ton and three quarter ton trucks. And since the vast majority of trucks purchased by consumers were half tons, limiting the emission constraints to them probably seemed to be a reasonable compromise to the government, while avoiding potential protests about cost and implementation by the truck manufacturers- -and their commercial customers- -that might have occurred had the laws been expanded to the heavier vehicles.

Before the advent of the catalytic converter, the distinction between “light”¯ and “heavy”¯ duty emission controlled trucks was largely ignored by consumers. But the converter’s introduction and it’s nearly universal application in 1975 “light duty”¯ emission vehicles changed things dramatically. Unlike the EGR and evaporative canister devices that preceded it, the converter had an exclusive appetite for expensive unleaded fuel which outraged truck buyers- -and especially fleet purchasers- -in a nation that still had access to relatively cheaper leaded fuel.

Recognizing a new marketing opportunity, or necessity, light truck makers made relatively minor spring, tire, and brake modifications to their existing half ton models to push GVWR’s just over the 6,000 lb. threshold. Ford lead the way by introducing the heavy half ton F150 as an alternative to their traditional half ton F100, while Chevrolet introduced the “F44 Heavy Duty Chassis”¯ package as an option for their C10 pickup.

These new-for-1975 models moved into the “heavy duty”¯ emission classification enjoyed by three quarter ton and heavier rated models. This allowed the F44 equipped C10, later dubbed the “Big 10”¯ for market visibility, to comply with government emissions regulations using only a PCV valve, heat stove, and relatively loose “controlled combustion system”¯ (CCS) tuning. In contrast, the 1975 “light duty”¯ emission certified standard capacity C10 required a PCV valve, heat stove, stricter “controlled combustion system”¯ (CCS) tuning, EGR, evaporative canisters, a vacuum activated early fuel evaporation (EFE heat riser) valve, special outside air ducting to the air cleaner, and a catalytic converter. But by 1979 the EPA- -following legislation enacted in California a year earlier- -raised the light duty/heavy duty emission dividing line from 6,000 to 8,500 lbs. GVWR which brought all heavy duty half ton and almost all three quarter ton pickups into the light duty emission fold. Following this legislation, consumer appreciation for the heavy duty half ton concept- -so gelded- -largely faded away.

A look at the chassis component differences that distinguish C10’s, Big 10’s, and C20’s:

What does this have to do with hauling around cement blocks? Well, let’s redirect the discussion to some component specifications for standard half ton C10’s, heavy duty half ton Big 10’s and three quarter ton C20’s. Note that the following pertains only to two wheel drive vehicles.

A careful study of the 1975 Chevrolet Light Truck Data Book provides some insight here. Leading into 1975 the regular (i.e. not heavy duty) C10 pickup offered several different GVWR packages. Those ratings were 4,900, 5,300, 5,400, 5,600, and 6,000 lbs. beyond this, the new F44 Heavy Duty Chassis package provided a 6,200 lb. GVWR. Mid-way through the model year, Chevy broadened the F44’s choices by adding a 6,050 rating option to the existing 6,200 lb. package. This late-availability 6,050 pound option replaced the C10’s 6,000 lb. package, which was cancelled.

All seven of these C10/ Big 10 GVWR packages used the same basic frame with a side rail width, depth, and thickness of 2.30”¯, 5.92”¯, and .156”¯ respectively. The frame section modulus was 3.14. Also, all of these packages utilized the GM 12 bolt semi floating axle with a capacity of 3,750 lbs., and used a standard 15”¯x6”¯ five lug wheel rim. Differences in GVWR were attributable to choices in spring and tire capacities, and brake system components. Some engine and transmission option recommendations/restrictions also accompanied different GVWR offerings.

The base 4,900 lb. rated C10 used the following components at minimum:
-   1,550 lb. capacity front springs.
-   1,550 lb. capacity rear springs (consisting of 4 leaves, and having a length of 52”¯ and a width of 2.5”¯).
-   Front and rear tires with a load capacity of 1,470 lbs.
-   Manual brake system (the 4,900 lb. rated C10 was the only pickup in the entire C/K10/20/30 lineup to use manual brakes. The front disk brakes had a diameter of 11.86”¯ and a thickness of 1.28”¯. Rear brake drums measured 11”¯x2”¯).
-   Standard LD4 250 1 bbl. I6 engine w/M15 Saginaw 3 speed manual transmission.

The 5,300 lb. GVWR required the following minimum additions and/or substitutions:
-   Option code J50 light duty power brake system (front disk and rear drum specifications remained the same as shown above).
-   Option code M20 Chevrolet CH465 4 speed manual transmission (recommended at 5,300 to 6,050 lb. GVW levels when LD4 250 1 bbl. I6 engine was selected; M15 3 speed manual transmissions remained available w/larger engines).

The 5,400 lb. GVWR required all items necessary for the 5,300 lb. rating plus the following minimum additions and/or substitutions:
-   Front and rear tires with a load capacity of 1,610 lbs.

The 5,600 lb. GVWR required all items necessary for the 5,400 lb. rating plus the following minimum additions and/or substitutions:
-   Option code G50 heavy duty rear springs (2,000 lb. capacity for this application. These springs consisted of 8 leaves, and were 56”¯ long x 2.5”¯ wide).
-   Front and rear tires with a load capacity of 1,790 lbs.

The 6,000 lb. GVWR required all items necessary for the 5,600 lb. rating plus the following minimum additions and/or substitutions:
-   Option code F60 heavy duty front springs (1,625 lb. capacity for this application).
-   Option code J55 heavy duty power brakes (this system replaced the lighter capacity J50’s single diaphragm brake booster with a dual diaphragm unit and included larger 11.15”¯x2.75”¯ rear drum brakes for C10 applications).

The 6,050 lb. GVWR package, which represents the lower of two F44 Big 10 capacities, was specifically identical, in terms of chassis and suspension components, to the 6,000 GVWR package which it superseded mid-year. Gross axle weight ratings at this GVWR were 3,100 lbs. in front and 3,580 lbs. in the rear, and are the same as those cataloged for the 6,000 lb. model. The 50 lb. increase in GVWR appears to be a result of “on paper only”¯ engineering. Emission control equipment was the only significant difference between the 1975 6,000 lb. and 6,050 lb. packages.

The 6,200 lb. GVWR was the maximum F44 option, and required all items necessary for the 6,050 lb. rating plus the following minimum additions and/or substitutions:
-   Front and rear tires with a load capacity of 1,900 lbs.
-   Option code LS9 350 4 bbl. V8 engine w/M15 Muncie 3 speed manual transmission.

A similar breakdown can be made for the C20 regular cab pickup, which was available for ‘75 with GVWR’s of 6,400, 7,100, 7,500, and 8,200 lbs. All four of these GVWR packages shared a frame with a side rail width, depth, and thickness of 2.30”¯, 5.92”¯, and .194”¯ respectively. While the side rail width and depth were the same as the C10/Big 10 frame, the thickness was greater, and in fact was the same as the thickness of the frame used in the one ton C30. Not surprisingly, the frame section modulus, at 3.92, was also more robust. Finally, all of these packages utilized the GM 14 bolt full floating axle with a capacity of 5,700 lbs., and used a standard 16.5”¯ eight lug wheel rim.

The base 6,400 lb. rated C20 used the following components at minimum:
-   1,750 lb. capacity front springs.
-   2,000 lb. capacity rear springs (consisting of 8 leaves, and having a length of 56”¯ and a width of 2.5”¯).
-   Front and rear tires with a load capacity of 1,990 lbs. (These used a 16.5x6”¯ rim.)
-   Power brake system with a dual diaphragm booster. (The front disk brakes had a diameter of 12.5”¯ and a thickness of 1.28”¯. Rear brake drums measured 11.15”¯x2.75”¯.)
-   Standard L25 292 1 bbl. I6 engine w/M15 Saginaw 3 speed manual transmission.

The 7,100 lb. GVWR required the following minimum additions and/or substitutions:
-   Option code G50 heavy duty rear springs (2,600 lbs. for this application. These springs consisted of 9 leaves and were 56”¯ long x 2.5”¯ wide).
-    Rear tires with a load capacity of 2,350 lbs.
-   Option code M20 Chevrolet CH465 4 speed manual transmission (recommended at 7,100 to 8,200 lb. GVW levels when L25 292 1 bbl. I6 engine was selected; M15 3 speed manual transmissions remained available w/larger engines).

The 7,500 lb. GVWR required all items necessary for the 7,100 lb. rating plus the following minimum additions and/or substitutions:
-   Front and rear tires with a load capacity of 2,780 lbs. (These used a 16.5x6.75”¯ rim.)
-   Option code J55 heavy duty power brakes (this system included larger 13”¯x2.5”¯ rear drum brakes for C20 applications).

The 8,200 lb. GVWR was the maximum rating available for the C20 and required all items necessary for the 7,500 lb. rating plus the following minimum additions and/or substitutions:
-   Option code F60 heavy duty front springs (1,900 lb. capacity for this application).
-   Option code G51 extra capacity rear springs (2,850 lbs.).

What they will carry:

To assess the load carrying ability and overall stamina of each of these C10/Big 10/C20 GVWR levels, let’s calculate the payload that each GVWR can handle. This requires subtracting the curb weight of the empty vehicle from the gross vehicle weight rating. To eliminate some of the variables bearing on curb weight calculations and thus facilitate comparisons between the different GVWR/payload packages and series, let’s assume that all vehicles referenced below are optionally equipped with the LS9 350 4 bbl. V8, M40 Turbo Hydra-matic 350 transmission, N41 power steering and an appropriately sized spare tire. (The Turbo 400 transmission was available only with the 454 engine option on these vehicles.)

The payload of a C10 Regular Cab Fleetside with 8’ box, 4,900 lb. GVWR, and the above options would be calculated as follows with vehicle and optional equipment weights taken from the 1975 Chevrolet Light Truck Data Book:

Gross vehicle weight rating (lbs.)    4,900
Curb weight (lbs.)    4,014
Payload (lbs.)    886 or 3 150 lb. people (using GM's favored methodology) and about 11 cement blocks at 40 lbs. apiece.

The payload of the same vehicle with a 5,300 lb. GVWR would be:

Gross vehicle weight rating (lbs.)    5,300
Curb weight (lbs.)    4,034
Payload (lbs.)    1,266 or 3 150 lb. people and about 20 cement blocks at 40 lbs. apiece.

The payload of the same vehicle with a 5,400 lb. GVWR would be:

Gross vehicle weight rating (lbs.)    5,400
Curb weight (lbs.)    4,047
Payload (lbs.)    1,353 or 3 150 lb. people and about 23 cement blocks at 40 lbs. apiece.

The payload of the same vehicle with a 5,600 lb. GVWR would be:

Gross vehicle weight rating (lbs.)    5,600
Curb weight (lbs.)    4,120
Payload (lbs.)    1,480 or 3 150 lb. people and about 26 cement blocks at 40 lbs. apiece.

The payload of the same vehicle with a 6,000 lb. GVWR would be:

Gross vehicle weight rating (lbs.)    6,000
Curb weight (lbs.)    4,148
Payload (lbs.)    1,852 or 3 150 lb. people and about 35 cement blocks at 40 lbs. apiece.

The payload of the same vehicle with the F44 package and 6,050 lb. GVWR would be:

Gross vehicle weight rating (lbs.)    6,050
Curb weight (lbs.)    4,124
Payload (lbs.)    1,926 or 3 150 lb. people and about 37 cement blocks at 40 lbs. apiece.

The payload of the same vehicle with the F44 package and 6,200 lb. GVWR would be:

Gross vehicle weight rating (lbs.)    6,200
Curb weight (lbs.)    4,142
Payload (lbs.)    2,058 or 3 150 lb. people and about 40 cement blocks at 40 lbs. apiece.

The payload of a C20 Regular Cab Fleetside with 8’ box, 6,400 lb. GVWR, and the options noted above would be calculated as follows

Gross vehicle weight rating (lbs.)    6,400
Curb weight (lbs.)    4,425
Payload (lbs.)    1,975 or 3 150 lb. people and about 38 cement blocks at 40 lbs. apiece.

The payload of the same vehicle with a 7,100 lb. GVWR would be:

Gross vehicle weight rating (lbs.)    7,100
Curb weight (lbs.)    4,463
Payload (lbs.)    2,637 or 3 150 lb. people and about 55 cement blocks at 40 lbs. apiece.

The payload of the same vehicle with a 7,500 lb. GVWR would be:

Gross vehicle weight rating (lbs.)    7,500
Curb weight (lbs.)    4,580
Payload (lbs.)    2,920 or 3 150 lb. people and about 62 cement blocks at 40 lbs. apiece.

The payload of the same vehicle with an 8,200 lb. GVWR would be:

Gross vehicle weight rating (lbs.)    8,200
Curb weight (lbs.)    4,594
Payload (lbs.)    3,606 or 3 150 lb. people about 79 cement blocks at 40 lbs. apiece.

Perhaps the most interesting comparison occurs between the C10 w/F44 6,200 lb. package, and the C20 w/6,400 lb. package. Despite the fact that the C20 has a 200 lb. heavier GVWR, its payload is actually 83 lbs. less than the Big 10’s! Of course, this is because the C20’s chassis is heavier by 283 lbs. But the parts that account for that extra weight offer significant benefits- -despite the Big 10’s slight advantage in ultimate payload capability, the C20’s stronger frame, full floating axle, and other more robust components suggest superior long term durability, particularly under near-maximum load conditions.

Moving beyond the 1975 numbers:

It is important to realize that the numbers I have selected above apply directly to 1975 model year vehicles. That said most of this information is applicable with only minor adjustment to C10’s, Big 10’s and C20’s produced through the end of the 1980 model year. In fact, the most significant deviation occurred in the C20 line for the 1980 model year, when the 8,200 lb. GVWR was replaced by an 8,600 lb. option under the C6P option code. Since the “light duty/heavy duty”¯ emission division point had been repositioned from 6,000 lbs. GVW to 8,500 lbs. GVW by start of the 1979 model year (1978 in California), the new for ‘80 C6P option again allowed buyers to use leaded gasoline in a three quarter ton truck. Of course, the 1979 and 1980 Big 10’s were equipped with the full complement of light duty emission controls.

This above discussion should also be helpful in contemplating the relative abilities of half tons and three quarter tons offered during the 1981 to 1987 model years although there was some minor reshuffling of GVW packaging. Having lost its “non-smog control”¯ appeal, the Big 10 package was no longer separately marketed from the lighter C10 models after 1980, although a maximum GVWR option of 6,100 lbs. was offered for C10’s from 1981 to 1987. Beyond this, some significant component changes occurred in the C20 lineup, such as the adoption of a semi floating axle for the first time in all three quarter tons (except those equipped with the 454 engine). Those changes might merit consideration in selecting one of the later models for your needs. But it is getting late, and that is beyond the scope of what I hoped to accomplish here.

Good luck as you consider your needs and choices!

Ken Lewis
Owner of a 1979 Chevy Big 10
« Last Edit: February 06, 2016, 10:42:59 AM by Irish_Alley »